1. Field of the Invention
This invention relates to an electronic tuning receiver using voltage-variable reactance elements in the radio-frequency tuning circuit and oscillation tuning circuit thereof, whereby tracking error is substantially eliminated over the receiving frequency band ranging from a low to a high receiving frequency.
2. Description of the Prior Art
In order to have a better understanding of the present invention, description will first be made of an example of conventional medium wave electronic tuning receiver shown in FIG. 1 of the drawings, wherein variable-capacitance diodes D.sub.1 and D.sub.2 of equal capacitance and adapted to serve as voltage-variable reactance elements are employed in a radio-frequency tuning circuit such as, for example, antenna tuning circuit 2, and an oscillation tuning circuit 3.
In FIG. 1, the antenna tuning circuit 2 comprising a tuning coil L.sub.1 and a capacitor C.sub.1 which are connected in parallel with each other; a DC blocking capacitor C.sub.P4 having one terminal thereof connected to one connection point between the tuning coil L.sub.1 and capacitor C.sub.1 ; and a variable-capacitance diode D.sub.1 having the cathode thereof connected to the other terminal of the DC block capacitor C.sub.P4, the anode of the variable-capacitance diode D.sub.1 being connected to the other connection point between the tuning coil L.sub.1 and capacitor C.sub.1, and grounded. The oscillation tuning circuit 3 comprises an oscillation coil L.sub.2 and capacitor C.sub.2 which are connected in parallel with each other; a padding capacitor C.sub.P5 having one terminal thereof connected to one connection point between the oscillation coil L.sub.2 and capacitor C.sub.2 ; and a variable-capacitance diode D.sub.2 having the cathode thereof connected to the other terminal of the padding capacitor C.sub.P5, the anode of the variable-capacitance diode D.sub.2 being connected to the other connection point between the oscillation coil L.sub.2 and capacitor C.sub.2, and grounded. Furthermore, a variable voltage source E is connected to the connection point between the variable-capacitance diodes D.sub.1 and the DC blocking capacitor C.sub.P4 through a resistor R.sub.0 and also to the connection point between the variable-capacitance diode D.sub.2 and the padding capacitor C.sub.P5 through a resistor R.sub.1. The variable voltage source E can be set up to a predetermined tuning voltage by means of a PLL synthesizer.
In conventional electronic tuning receiver shown in FIG. 1, a tuning voltage is applied to the variable-capactance diodes D.sub.1 and D.sub.2 to select a desired frequency. However, such a superheterodyne receiver is disadvantageous in that tracking error inevitably occurs from a logical point of view.
FIG. 2 shows the relationships between the tracking error and receiving frequency which occur with a medium wave band receiver such as shown in FIG. 1, wherein tracking error curve (A) indicates tracking error which tends to occur with the conventional receiver, from which it will be seen that it is at only three points b, d and f, i.e., at the points of a low receiving frequency (600 KHz), medium receiving frequency (1000 KHz) and high receiving frequency (1400 KHz) tracking error becomes zero, and that at the other receiving frequency points, tracking error 5 to 10 KHz at a maximum occurs.
The aforementioned medium wave band receiver includes factors to deteriorate the receiving sensitivity. With an AM stereo receiver, for example, there is a tendency that variations in stereo separation degree and side band level, phase shift and so forth are caused so that the broadcast quality is remarkably degraded. Thus, it has so far been desired that tracking error be eliminated.
Furthermore, in the conventional medium wave band receiver, an extremely great tracking error occurs at a receiving frequency in the vicinity of 1700 KHz, and thus with such a receiver using the conventional electronic tuning circuit as it is, problems arise from the standpoint of practical use.